G. Ya. Fraikin, M. G. Strakhovskaya*, and A. B. Rubin

Received June 25, 2013
Progress in understanding primary mechanisms of light reception in
photoregulatory processes is achieved through discovering new
biological photoreceptors, chiefly the regulatory sensors of blue/UV-A
light. Among them are LOV domain-containing proteins and DNA
photolyase-like cryptochromes, which constitute two widespread groups
of photoreceptors that use flavin cofactors (FMN or FAD) as the
photoactive chromophores. Bacterial LOV domain modules are connected in
photoreceptor proteins with regulatory domains such as diguanylate
cyclases/phosphodiesterases, histidine kinases, and DNA-binding domains
that are activated by photoconversions of flavin. Identification of
red/far-red light sensors in chemotrophic bacteria
(bacteriophytochromes) and crystal structures of their photosensor
module with bilin chromophore are significant for decoding the
mechanisms of phytochrome receptor photoconversion and early step
mechanisms of phytochrome-mediated signaling. The only UV-B regulatory
photon sensor, UVR8, recently identified in plants, unlike other
photoreceptors functions without a prosthetic chromophore: tryptophans
of the unique UVR8 protein structure provide a “UV-B
antenna”. Our analysis of new data on photosensory properties of
the identified photoreceptors in conjunction with their structure opens
insight on the influence of the molecular microenvironment on
light-induced chromophore reactions, the mechanisms by which the
photoactivated chromophores trigger conformational changes in the
surrounding protein structure, and structural bases of propagation of
these changes to the interacting effector domains/proteins.
KEY WORDS: phytochromes, cryptochromes, LOV-domain photosensors,
chromophores, structure, signaling